Overview of the Special Issue “Mechanism of Phreatic Eruptions and Challenges for Eruption Forecasting: Latest Advances and Volcanic Disaster Prevention”

Owakudani, the eruption center of the 2015 eruption of Hakone Volcano, is one of the most popular tourist destinations in Japan, attracting more than three million visitors every year. However, the new crater created by the eruption is only 200 m from an area where tourists stroll (e.g., Mannen et al., 2018). Moreover, several craters that are assumed to have been the sources of ancient phreatic eruptions much larger than the 2015 eruption have been recognized near Owakudani using topographic and geological analyses (e.g., Yamaguchi et al., 2021). On the other hand, a slight inflation of the volcanic edifice and an increase in seismic activity, as observed before the 2015 eruption, are not rare in Hakone volcano. Indeed, they have been occurring once every few years since 2001. Because sightseeing at Owakudani is valuable for the local economy, it is not practical to designate the area as restricted every time seismic activity increases. Consequently, it is important to understand the processes of volcanic eruptions and to evaluate the imminence of an eruption in order to minimize economic losses, while ensuring the safety of tourists and residents. In addition to analyzing the 2015 eruption, the Hot Springs Research Institute of Kanagawa Prefecture, which has been engaged in research on Hakone volcano for many years, has been surveying the latest studies related to phreatic eruptions to consider the direction of future research on volcano monitoring. Activities include publishing a special issue in an international journal named Earth, Planets and Space (Mannen et al., 2019) and hosting domestic and international workshops on phreatic eruptions (Abe, 2019; Man nen and Kato, 2020). This special issue contains presentations given at the International Workshop on Phreatic Eruption Mechanisms, which was co-organized with the Kanagawa Prefectural Museum of Natural History in January 2020 with support from local companies and organizations, the Town of Hakone, and the Tokyo Geographical Society, as well as papers based on subsequent research. The contents of the special issue are as follows. Yukutake and Mannen (2021) summarize the latest research on hydrothermal systems, which are largely responsible for the occurrence of phreatic eruptions. In this review, they focus on the formation of low-permeable layers in hydrothermal systems. The shallow impermeable layer is formed of clay minerals generated by hydrothermal alteration, and can be detected with magnetotelluric surveys as low resistivity bodies. The deeper impermeable layer is thought to be formed by silica precipitation, 地学雑誌 Journal of Geography(Chigaku Zasshi) 130(6)719­723 2021 doi:10.5026/jgeography.130.719